The circumstellar dust shell of the post-AGB star HD 161796

We have modeled the complete optical to millimeter spectrum of the Post-Asymptotic Giant Branch (Post-AGB) star HD 161796 and its circumstellar dust shell. A full 2-200 mum spectrum taken with the Infrared Space Observatory was used to constrain the dust properties. A good fit is achieved using only 4 dust components: amorphous silicates, the crystalline silicates forsterite and enstatite, and crystalline water ice, contributing respectively about 63, 4, 6 and 27% to the total dust mass. The different dust species were assumed to be co-spatial but distinct, resulting in different temperatures for the different grain populations. We find a temperature for the crystalline H2O ice of 70 K, which is higher than thermal equilibrium calculations of pure H2O ice would give. This implies that the ice must be formed as a mantle on top of an (amorphous) silicate core. In order to form H2O ice mantles the mass loss rate must exceed some 5 x 10(-5) M-. yr(-1). With a water-ice fraction of 27% a lower limit for the gas to dust mass ratio of 270 is found. At a distance of 1.2 kpc (Skinner et al. 1994) and adopting an outflow velocity of 15 km s(-1) (Likkel et al. 1991) an AGB mass loss rate of (5.1 x 10(-4) M-. yr(-1)) is found, which lasted 900 years and ended 430 years ago. During this phase a total of 0.46 M-. was expelled. The mass loss rate was high enough to account for the presence of the H2O ice.